[Quantization] Consolidate experts_int8 with fp8 online quantization

[Josephasafg]

Purpose

Following up on this #38032 - This PR Consolidates experts_int8 with fp8's online quantization infrastructure (QeRL). Extracts shared online MoE quantization logic into a common base class and refactors fp8's MoE kernel infrastructure into a reusable mixin.

Test Plan

experts_int8 and fp8 tests should pass

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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[gemini-code-assist]

Code Review

This pull request refactors the online MoE quantization infrastructure by introducing a common base class, OnlineMoEMethodBase, and a mixin, Fp8MoEKernelMixin, to share logic between different quantization methods. It migrates ExpertsInt8MoEMethod and Fp8OnlineMoEMethod to this new architecture, which utilizes meta-device weight allocation and deferred quantization after model loading. Review feedback identifies potential division-by-zero issues in the int8 quantization loop when encountering zero-valued weight rows and highlights inefficient cross-device tensor allocations for scale parameters that should be created on the same device as the weights.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Josephasafg.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.